Maximoff was an enhanced individual, after having been forced to be a test subject on and receiving super speed. His twin sister, Wanda (Scarlet Witch), was also tested on and given hypnotic and telekinetic abilities. Their captor, Baron Wolfgang von Strucker, kept them locked up in Sokovia, until the Avengers stormed his base, inadvertently setting them free. They were then mislead by Ultron into taking down the Avengers with him, but Wanda read his mind (while he was downloading himself to a new body attached with the Mind Stone) and discovered that he planned to eradicate humanity.
The twins turned against him and joined the battle with the Avengers at Sokovia, where Ultron planned to use a vibranium machine to cause global extinction. During the battle, Pietro died saving Hawkeye and a child.
In the credits and comics, Maximoff is codenamed Quicksilver, but he's never actually referred to by that name by another character or anywhere else on-screen.
- Captain America: The Winter Soldier (2014, mid-credits scene)
- Avengers: Age of Ultron (2015)
- Captain America: Civil War (2016, photo of young Pietro)
- Joss Whedon filmed alternate ending was filmed, where Maximoff survived and was officially recruited as a new member of the Avengers, in case Disney opposed his death.